This invention relates generally to orthopedic surgical devices and particularly to fixation screws for stabilizing factured bones.
External fixation devices using pins extending into or through bone fragments are generally accepted and widely used throughout the world. The present invention provides an improved fixation screw suitable for use with conventional external devices.
Early fixation pins were smooth, cylindrical shafts which were passed through pre-drilled holes. These pins made no screw and thread type engagement to the bone.
Later, pins were developed which were self-drilling and self-tapping. These pins were smooth, cylindrical shafts with the points matched into a pointed spade configuration which formed the drill tip, having two flat inclined surfaces on opposite sides of the longitudinal axis, and a point wedge-shaped, spade surface with knife edges that scraped away the bone when the shaft was turned.
On the self-drilling, self-tapping pin, a self-tapping thread was started at a point approximately half-way up the shallow sloping surfaces. This self-tapping thread continued up the shaft for a distance sufficient to pass through the bone for which the pin was designed.
Self-drilling, self-tapping screws have several disadvantages. First, the knife edge of the drill point is not very sharp. Consequently, the drill advances at a relatively slow speed through the bone. This slow speed was generally slower than the speed with which the self-tapping thread would advance if the hole were pre-drilled before attempting to tap the pin into the bone. This speed differential caused the thread portion to strip out the threads just cut in the bone because of the inability to advance as fast as the self-tapping thread would normally advance.
A second disadvantage is that the relatively slow speed of drilling achieved by this structure resulted in higher temperatures from friction heating of the bone surrounding the hole. Since bone is a living structure, it dies when overheated. Clinical testing has shown that when bone cells are heated to a temperature of about 105.degree. Fahrenheit, they die. As a result, after such a pin was placed in the bone, often a small plug of bone around the hole would subsequently die. As a result, it frequently happened that a small plug of bone with the pin attached would fall out. New and painful treatment was thus necessitated for the patient.
Self-penetrating and pre-drilled screws are known. The self-penetrating types generally have a sharp point on the tip of the screw similar to self-penetrating wood screws. These self-penetrating screw structures could not be used in orthopedic work because the hardness and thickness of the bone portion surrounding the marrow would not permit the screw to penetrate absent a drilling point on the tip of the screw.
In fixing a broken bone with a fixation screw, the orthopedic surgeon must pass a fixation screw through the bone table on one side of the marrow, pass the screw through the marrow and then must find the hole in the bone table on the other side of the marrow with the tip of the screw. All of these procedures must be done by feel with the physician unable to see the holes drilled in the bone. Since the interior surfaces of the bone table facing the marrow tend to be porous, if a sharp point were used on the far side of the marrow would be more difficult because of the difficulty of sliding a sharp point over a porous surface. Generally pre-drilled, self-tapping screws utilize truncated conical surfaces at their points. These truncated surfaces have sharp edges which could catch on the porous internal bone surfaces facing the marrow when attempting to slide the fixation pin over the surface in search of the far-side hole.
In addition, such pre-drilled, self-tapping screws generally utilize only one flute cut in the tip to give a cutting edge to the helical rib of the thread. Where only one flute is used, there are unbalanced cutting forces generated as the helical rib cuts a helical groove in the bone. These forces would create very large pressures pressing the screw shaft against the bone at a point diagonally opposite the flute. These large pressures could cause damage to the bone material. Hence, the use of a structure causing balance cutting forces is preferable.
Placing a large screw in a predetermined position in a bone presents added difficulties because the point of the screw tends to slide across the outer surface of the bone. In many applications, it is difficult to ascertain the exact location of the tip of a large bone screw.